Intraocular pressure measurement system including a sensor mounted in a contact lens

ABSTRACT

An apparatus for measuring intraocular pressure (IOP) comprises a contact lens including an inner surface contoured to a surface portion of an eye and a sensor disposed in the contact lens. The sensor comprises a contact surface for making contact with the surface portion of the eye. The contact surface includes an outer non-compliant region and an inner compliant region fabricated as an impedance element that varies in impedance as the inner compliant region changes shape. The sensor further comprises a region of conductive material electrically coupled to the impedance element of the compliant region and responsive to an external signal for energizing the impedance element so that the IOP may be determined.

RELATED APPLICATIONS

This application is a continuation-in-part of a co-pending U.S. patentapplication Ser. No. 09/642,573, entitled “SYSTEM FOR MEASURINGINTRAOCULAR PRESSURE FOR AN EYE AND A MEM SENSOR FOR USE THEREWITH”,filed Aug. 21, 2000 now U.S. Pat. No. 6,447,449. The subject matter ofthe aforementioned application is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system for measuring intraocularpressure (IOP) in an eye, and is particularly directed to a system formeasuring IOP that utilizes a sensor fabricated throughmicroelectromechanical system (MEMS) technology and which is mounted ina contact lens.

BACKGROUND OF THE INVENTION

Glaucoma patients and post-operative patients of eye surgery requireregular monitoring of the IOP of their eyes in order to diagnosedegenerative conditions which may lead to degraded sight and/orblindness without immediate medical treatment. Accordingly such patientsmust make frequent trips to their ophthalmologist's office for thisregular monitoring of their IOP with conventional mechanical impact typetonometers. This becomes a nuisance to the patient after a time leadingto patient resistance to compliance. In addition, the only measurementof the patient's IOP that the doctor can use for diagnosis is thepressure that exists at the time of the office visit. Therefore, if thepressure is normal at the time of the visit, but becomes highthereafter, the patient's actual risk of blindness may be misdiagnosed.Also, if the pressure measured at the time of the office visit is highfor reasons other than eye degeneration, the patient may be falselydiagnosed and be required to undergo therapy that may not be needed.

Intraocular pressure has been known to fluctuate widely during any givenperiod of time and thus, should be monitored many times during theperiod of a day in order to gain an average or representative IOP whichin turn may be tracked for diagnosis. Attempts have been made to permitglaucoma patients to monitor their IOP at home many time during theperiod of a day with a self-tonometry portable instrument. Reference ismade to the paper “Self-Tonometry to Manage Patients with Glaucoma andApparently Controlled Intraocular Pressure”, Jacob T. Wilensky et al.,published in Arch Ophthalmol, Vol. 105, August 1987 for more details ofsuch a device. This paper describes a portable, tonometer instrumentconsisting of a pneumatically driven plunger, fitted with an elasticmembrane, that slowly comes forward and applanates the cornea.Applanation is detected by an internal optic sensor and the pressurenecessary to achieve applanation is registered and displayedautomatically. The patient is able to prepare the eye and self-tonometerand activate the instrument for taking the measurement. However, thedevice proposed is relatively large and bulky, about the size of anattache' case, for example, and not conducive to convenient transportwith the patient during normal daily routine in order to measure IOP. Inaddition, the proposed technique requires special eye preparation byinstilling a topical anesthetic in the eye prior to tonometricmeasurements.

Also, very crude attempts have been made to develop methods ofnon-invasively monitoring IOP using passive electronic circuitry andradiotelemetry disposed at the eye. In the papers of R. L. Cooper et al.namely, those published in Invest., Ophthalmol Visual Sci., pp. 168-171,February 1977; British JOO, 1979, 63, pp. 799-804; Invest, OphthalmolVisual Sci., 18, pp. 930-938, September, 1979; and Australian Journal ofOphthalmology 1983, 11, pp. 143-148, a miniature guard ring applanatingtranssensor (AT) which included electronic components that changed inresonance proportional to the IOP was mounted in an acrylic or sauflonhaptic contact lens element that was individually designed for the humaneye. The AT was mounted in the lower part of the scleral haptic so thatit applanated the inferior sclera under the lower lid. The whole hapticring was placed in the conjunctival fornix. IOP was monitored from theAT with an automatic continual frequency monitor (ACFM) attached byadhesive and elastic bands to the exterior of the lower eye lid. TheACFM induced in the AT electromagnetic oscillations at varying radiofrequencies via a magnetic coupling of inductive coils and monitored forits resonant frequency representative of IOP. This device is clearlyuncomfortable and bulky, minimizing expected patient compliance. Inaddition, the device measures IOP by applanation of the sclera, which isa rather unconventional method of measuring IOP.

In another paper reported in Investigative Ophthalmology Reports, pp.299-302, April, 1974 by B. G. Gilman, a device is presented formeasuring IOP of a rabbit in a continuous manner with strain gaugesmounted (embedded) in soft flush fitting, silastic gel (hydrogel)contact lenses. The exact shape of the eye of the rabbit was obtained bya molding procedure. Leads of the strain gauges extended from the lensand were connected to a wheatstone bridge arrangement for measurementtaking. The paper suggests that the embedded strain gauges may be usedwith a miniature telemetry package completely contained in a hydrophilichydrogel contact lens for continuous, noninvasive, long durationmonitoring of IOP, although no design was provided. This device proposeswire connections for telemetry which entails wires to be run out of theeye under the eyelid. Also, the proposed approach requires the moldingof a special contact for each individual eye, a practice which wouldmake widespread use unattractive and expensive.

In 1993, an IEEE paper was presented by C. den Besten and P. Bergveld ofthe University of Twente, The Netherlands, proposing a new instrumentfor measuring area of applanation entitled “A New Tonometer Based onApplication of Micro-Mechanical Sensors”. This new instrument is basedon the Mackay-Marg principle of tonometer operation in which a platehaving a diameter of 6 mm or less is pressed against and flattens aportion of the cornea of the eye, referred to as “applanation”. In themiddle of the plate is a small pressure sensitive area that is pressedagainst the flattened portion of the cornea with a slowing increasingforce while the pressure area is electronically measured. Theapplanation sensor of this new instrument comprises a micro-machinedplunger and pressure sensing electronics on three electrically insulatedlevels of a silicon substrate resulting in a modified Mackay-Margtonometer in which the radius of the flattened area and the distancebetween the periphery of the applanation and the pressure center can bemeasured to render a more accurate pressure area measurement. In thework presented in this paper, the researchers did not actually propose apressure sensor or transducer. In addition, it is not clear if, for aslong as the eye is applanated, there is a need to know the area ofapplanation. Sufficient applanation is usually determined by thedifference in trough height from the peak to dip of the pressureprofile. The dip is unlikely to occur unless sufficient applanation isachieved.

Also, in the U.S. Pat. No. 5,830,139 entitled “Tonometer System forMeasuring Intraocular Pressure by Applanation and/or Indentations”,issued to Abreu on Nov. 3, 1998, a tonometer system is disclosed using acontact device shaped to match the outer surface of the cornea andhaving a hole through which a movable central piece is slidably disposedfor flattening or indenting a portion of the cornea. A magnetic fieldcontrols the movement of the central piece against the eye surface toachieve a predetermined amount of applanation. A sophisticated opticalarrangement is used to detect when the predetermined amount ofapplanation has been achieved to measure IOP and a calculation unitdetermines the intraocular pressure based on the amount of force thecontact device must apply against the cornea in order to achieve thepredetermined amount of applanation. The magnetic and opticalarrangements of this device requires special alignment and calibrationtechniques rendering it difficult for use as a self-tonometry device.

While the various foregoing described U.S. patent and papers proposevarious devices and instruments for tonometry, none appears to offer aviable inexpensive, convenient solution to the immediate problem ofself-tonometry. The present invention overcomes the drawbacks of theproposed instruments described above to yield a simple, inexpensive andeasy to use instrument that completely automates the tonometry processand offers post-processing of tonometer IOP readings from which a properelevation and diagnosis by an ophthalmologist may be performed.

SUMMARY OF THE INVENTION

The present invention is an apparatus for measuring intraocular pressureof an eye. The apparatus comprises a contact lens including an innersurface contoured to a surface portion of the eye and a sensor disposedin the inner surface of the contact lens. The sensor comprises a contactsurface for making contact with the surface portion of the eye. Thecontact surface includes an outer non-compliant region and an innercompliant region fabricated as an impedance element that varies inimpedance as the inner compliant region changes shape. The sensorfurther comprises a region of conductive material that is electricallycoupled to the impedance element of the compliant region and responsiveto an external signal for energizing the impedance element so that theintraocular pressure may be determined.

The present invention also provides a method for measuring intraocularpressure (IOP) of an eye. According to the inventive method, a contactlens is provided with an inner surface contoured to the eye. The contactlens includes a sensor disposed in the inner surface of the contactlens. The sensor has a compliant region that functions as an impedanceelement. The contact lens is positioned on the surface portion of theeye. An applanator is provided for applying pressure against the contactlens. The applanator is moved toward the eye until the sensor forcefullyengages the surface portion of the eye which causes the compliant regionto change shape and vary in impedance. The impedance element isenergized and a representative pressure measurement is determined eachtime the impedance element is energized. The representative pressuremeasurements are processed to render a resultant IOP measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a cross-sectional view of a first embodiment of a tonometersensor for use in the present invention;

FIG. 2 is a plan view of the tonometer sensor of FIG. 1;

FIGS. 3A and 3B are cross-sectional and plan views, respectively, of thetonometer sensor illustrating additional regions in accordance with thepresent invention;

FIGS. 4A and 4B are cross-sectional and plan views, respectively, of atonometer sensor constructed in accordance with an alternate embodimentof the present invention;

FIG. 5A is a graph illustrating the relationship between deflection ofthe tonometer sensor and intraocular pressure (IOP);

FIG. 5B is a graph illustrating the relationship between resonantfrequency of the tonometer sensor and IOP;

FIGS. 6(a 1)-6(i 2) are cross-sectional and plan views, respectively, ofthe tonometer sensor through various stages of a fabrication process;

FIGS. 7(a 1)-7(j 2) are cross-sectional and plan views, respectively, ofan alternate tonometer sensor through various stages of a fabricationprocess;

FIGS. 8(a 1)-8(d) are cross-sectional and plan views of anotheralternate tonometer sensor through various stages of a fabricationprocess;

FIG. 9 is a side illustration of an apparatus for measuring IOP of aneye using the tonometer sensor of FIG. 3;

FIG. 10A is a sectional view taken along line 10A—10A in FIG. 9 withparts omitted for clarity;

FIG. 10B is a sectional view taken along line 10B—10B in FIG. 9 withparts omitted for clarity;

FIGS. 11A1—11E2 are illustrations of the response of the apparatus ofFIG. 9 to contact with an eye;

FIG. 12 is a functional block diagram schematic of a control unit foruse with the apparatus of FIG. 9;

FIG. 13 is an illustration of an apparatus for measuring IOP inaccordance with an alternate embodiment; and

FIG. 14 is a sectional view taken along line 14—14 in FIG. 13.

DETAILED DESCRIPTION OF EMBODIMENTS

A tonometer sensor 10 produced using microelectromechanical system(MEMS) techniques is shown in FIGS. 1 and 2. The tonometer sensor 10includes a substrate 12 that is comprised of a silicon material, but itshould be understood that other materials may be used. The substrate 12includes a contact surface 14 for making contact with a surface portion34 (FIG. 3A) of an eye 36. The contact surface 14 includes an outernon-compliant region 16 (FIG. 1) and an inner compliant region 18 thatis fabricated using MEMS techniques (which will be described in greaterdetail herein below) as an impedance element, the impedance of whichvaries as the inner compliant region 18 changes shape. The compliantregion 18 comprises a diaphragm 20 as one plate of a capacitive elementthat is separated by a dielectric 22 from another plate 24 of thecapacitive element which is part of the non-compliant region 16. As willbecome more evident from the description below, as the contact surface14 is pressed against the surface portion of the eye, the diaphragmplate 20 flexes closer to the other non-compliant plate 24 to change thecapacitance of the capacitive element in proportion to the intraocularpressure (IOP) of the eye. In the illustrated embodiment, the dielectriccomprises air, but other suitably compliant dielectrics such as hydrogeland silicone, for example, may also be used, without deviating from theprinciples of the present invention.

As shown by the substrate cross-sectional and plan views of FIGS. 3A and3B, respectively, a region of conductive material 38 is included as partof the substrate 12 and is electrically coupled to the impedance elementof the compliant region 18 (diaphragm 20) which is a capacitive element.While not shown in FIGS. 3A and 3B, this electrical coupling isdescribed in greater detail in connection with the fabrication drawingsfound herein below. The conductive material 38 is responsive to anexternal signal for energizing the impedance element so that the IOP maybe determined. In FIGS. 3A and 3B, the conductive region 38 comprises aninductor coil fabricated in the non-compliant region 16 of the contactsurface 14 such that it is electrically coupled to the capacitiveelement to form a resonance or tank circuit. It should be understoodthat other types of sensors (piezoelectric, piezoresistive, strain-gagebased, etc.) could be substituted for the sensor 10. Such other types ofsensors would likely require use of other known telemetry techniquesrather than a tank circuit.

In the present embodiment, the inductor coil 38 is formed by disposingconductive material in a predetermined pattern, like a concentricspiraled pattern, for example, in the non-compliant region 16. A processfor fabricating the inductor coil 38 at the non-compliant region 16 isdescribed in greater detail herein below. However, it should beunderstood that the inductor region need not be embodied solely at thenon-compliant region 16 and may be embodied as part of the compliantregion 18 as well without deviating from the principles of the presentinvention. Further, it should be understood by those of ordinary skillin the art that there could be a spiral inductor 42 on the contactsurface 14 of the diaphragm 20 coupled to a flat spiral inductor 44underneath the diaphragm as illustrated in the alternate embodiment ofFIGS. 4A and 4B. Yet another alternative would include a combination ofthe aforementioned spiral inductor 42 and the capacitive element, formedby the diaphragm (plate) 20 and the fixed plate 24, acting inconjunction with each other, meaning the inductance and the capacitancewill increase (as the plates get closer to each other) or decreasetogether.

In the present embodiment, the resonant circuit comprising the inductorcoil 38 and the capacitive element formed by the plates 20 and 24 may beexcited into resonance by an external electromagnetic signal in theradio frequency (RF) range. Tank circuits of this type have a naturalresonant frequency fo that, to the first order, depends of the values ofthe inductor and the capacitor as follows:

fo=½π(LC)^(1/2)

where L is the inductance and C is the capacitance. Accordingly, as thecapacitance of the tonometer sensor 10 changes, the resonant frequencyfo of the tank circuit will change in proportion thereto.

For example, if the contact area 14 of the tonometer sensor 10 isapproximately one square millimeter (1 mm²) or one millimeter (1 mm) oneach side, the diaphragm 20 of the compliant region 18 may have adiameter of five hundred micrometers (500 μm) with a one and a halfmicrometer (1.5 μm) dielectric or air gap, and the inductor coil mayhave twenty-five (25) turns with an inside diameter (ID) of five hundredmicrometers (500 μm) and an outside diameter (OD) of one thousandmicrometers (1,000 μm) With the diaphragm 20 undisturbed, the resonantfrequency may be on the order of one hundred and ninety-three megahertz(193 MHz). Accordingly, a ten percent (10%) increase in capacitance, forexample, resulting from a diaphragm 20 deflection will produce adownward shift in resonant frequency to one hundred and eighty-fourpoint one megahertz (184.1 MHz) and this shift in resonant frequency isreadily discernible electronically as will be further described hereinbelow. It is understood that the contact area of the sensor 10 may beless than 1 mm, in which case the various dimensions may be rescaledproportionately.

As has been described in connection with the illustration of FIG. 3A,the deflection of the diaphragm 20 of the compliant region 18 as thecontact surface 14 of the substrate 12 is pressed against the surfaceportion 34 of the eye 36 is representative of the IOP of the eye. Thegraph of FIG. 5A illustrates an exemplary center deflection inmicrometers (μm) expected for a diaphragm 20 with the geometry describedabove as a function of the IOP of the eye expressed in parametric unitsof millimeters of Mercury (mm Hg). It is this deflection of thediaphragm 20 which causes the change in capacitance and may be measuredby the resultant change in resonant frequency of the tank circuit. Thegraph of FIG. 5B illustrates an estimated change in resonant frequencybased upon a conservative approximation of a corresponding change incapacitance resulting from the deflection of the diaphragm 20 due toIOP. The expression of resonant frequency (MHz) to IOP (mm Hg)illustrated by the graph is nonlinear as expected in a capacitivesensing structure for measuring IOP.

An exemplary process suitable for fabricating an embodiment of thetonometer sensor 10 is shown in the process diagrams of FIGS. 6(a 1)through 6(i 2) wherein each Figure provides cross-sectional and planviews, respectively, of the sensor structure at various stages of thefabrication process. The process starts with a substrate 100 which maybe part of a silicon wafer, for example, as shown in FIG. 6(a). It isunderstood that materials other than silicon may be used for thesubstrate in which case the process may be slightly modified toaccommodate such other material. The substrate has a top surface 102 anda bottom surface 104. In the step of FIG. 6(b), an etch resistant layeris provided over the substrate, like silicon dioxide (SiO₂), forexample, and the top surface 102 is patterned using conventionallithograph/etching processes to form the capacitor well region 106having a diameter of approximately 500 μm, for example, and spiraledgroove regions 108 of a width on the order of 5 μm, for example, for theinductor coil. Thereafter, the unpatterned etch resist areas of the Sisubstrate are etched using a deep etch process, like reactive ionetching, for example, to a depth of one to twenty microns and the etchresist is removed rendering a structure as shown in FIG. 6(b).

In the step of FIG. 6(c), a layer of silicon nitride (Si₃N₄) or othersimilar material 110 is deposited on the surfaces of the substrate 100.A conformal coating of Si₃N₄ is deposited over the surface of thesubstrate through a conventional chemical vapor deposition (CVD) processto a thickness of approximately 1200 Å-2400 Å, for example. Next, in thestep of FIG. 6(d), a layer of low temperature oxide (LTO) 112 isdeposited over the Si₃N₄ layer 110 by conventional CVD to a thickness ofapproximately 2-3 μm, for example. The LTO layer 112 of the top surface102 is polished smooth using a chemical mechanical polishing process,for example, and patterned using a conventional photolithography processto form an anchor region 114 which, for the present embodiment, is inthe form of an annulus of a width of approximately 50-100 micronssurrounding the capacitive well region 106. The anchor region 114 isetched through the LTO layer 112 down to the Si₃N₄ layer 110 using areactive ion etching process, or a wet etching process using bufferedhydrofluoric acid (BHF), or other similar process.

In the step of FIG. 6(e), a layer of polysilicon 118 is deposited,preferably by CVD, over the surface of the LTO layer 112 of FIG. 6(d)and the layer of polysilicon at the top surface 102 is patterned andetched in a conventional manner to form an unetched layer of polysilicon120 covering substantially the capacitive well region 106 and anchoredby region 114 to the nitride layer. A hole 122 may be provided throughan edge of the polysilicon layer 120 to the LTO and Si₃N₄ layers 112 and110 thereunder by the aforementioned patterning and etching process ofFIG. 6(e). A post annealing process is performed to render the membranesection of polysilicon 120 in tension. In the present embodiment, thestructure of FIG. 6(f) is put in an oven and heated for approximately 30minutes at approximately 900° C. which changes the crystalline makeup ofthe polysilicon to provide for stress modification thereof.

In the step of FIG. 6(f), the LTO and nitride layers 112 and 110,including the layers under the polysilicon layer 120, are removed,preferably by a conventional BHF etching process wherein the BHF isallowed to flow through the hole 122 and etch the LTO and nitride layersunder the polysilicon layer 120 which are released in solution throughthe same hole 122. Accordingly, a polysilicon diaphragm 120 in tensionis produced as shown in FIG. 6(f). Next, the hole 122 in the polysilicondiaphragm is sealed by growing a low temperature oxide layer (not shown)over the hole 122 in a conventional furnace environment.

In the step of FIG. 6(g), the grooved areas 108 may be pretreated toaccept a conductive material which may be deposited in the grooves byconventional plating, sputtering or evaporation techniques, for example,to form the inductor coil 124. Metals which may be used for this processinclude Ni, Au, Fe, Ag, and Pt to name a few. Preferably, the metallicplating is performed electroless, but electroplating may also be usedwithout deviating from the principles of the present invention.

As shown in FIG. 6(h), interconnects 126 and 128 are provided from theends of the inductor coil 124 to corresponding sides of the capacitiveelement. For the interconnect region 126, a window is formed in thenitride layer 110 between the conductive material of the inside coil 130and the polysilicon layer 120 which is one side of the capacitiveelement of the sensor 10. When the window region is plated, the metalend 130 of the inductor coil 124 will make electrical contact with oneside 120 of the capacitive element. For the interconnection region 128,a window is formed in the nitride layer 110 between the substrate andthe groove of the other end 132 of the coil 124 such that when plated,metal electrically connects the other end 132 of the coil 124 with thesilicon substrate 100, which is the other side of the capacitiveelement, thus, completing the tank or oscillatory circuit. In the stepof FIG. 6(i), a thin layer of non-conducting material 136 may be grownover the metallic plated surfaces of the non-compliant region 16 toensure against the sections of the inductor coil 124 making contact witheach other over the surface of the nitride layer 110.

An embodiment for illustrating a fabrication process of an alternateembodiment of the tonometer sensor 10 is shown in the FIGS. 7(a 1)through 7(j 2) wherein each Figure provides cross-sectional and planviews, respectively, of the alternate sensor structure at various stagesof the fabrication process. The process starts with a substrate 140which may be part of a silicon wafer, for example, as shown in FIG.7(a). It is understood that materials other than silicon may be used forthe substrate in which case the process may be slightly modified toaccommodate such other material. The substrate 140 has a top surface 142and a bottom surface 144. In the step of FIG. 7(b), a layer of siliconnitride (Si₃N₄) or other similar material 146 is deposited on the topand bottom surfaces 142 and 144 of the substrate 140. In the presentembodiment, the Si₃N₄ 146 is deposited through a conventional chemicalvapor deposition (CVD) process to a thickness of approximately 1200 Å,for example.

Next, in the step of FIG. 7(c), a layer of low temperature oxide (LTO)148 is deposited over the Si₃N₄ layer 146 by conventional CVD to athickness of approximately 1.5 μm, for example. The LTO layer 148 of thetop surface 142 is patterned using a conventional photolithographyprocess to form a circled region 150 having a diameter of approximately500 μm, for example, on top of the Si₃N₄ layer 146, and the unpatternedregions 152 around the circled region 150 and on the bottom surface 144are etched using a reactive ion etching process or a wet etching processusing buffered hydrofluoric acid (BHF), or other similar process.

The top surface 142 of the resulting structure as shown in FIG. 7(d) isdeposited with another low temperature oxide layer, preferably by CVD,to a thickness of approximately 0.5 μm, for example. This second LTOlayer 154 is patterned and etched in a conventional manner such that theremaining unetched second LTO layer overlaps the circled layer 150concentrically to form an annular region of approximately 50 μm on topof the Si₃N₄ layer 146 surrounding the circled region 150 as shown inFIG. 7(e).

In the step of FIG. 7(f), a layer of polysilicon is deposited,preferably by CVD, over the top surface 142 of the structure of FIG.7(e), and the layer of polysilicon is patterned and etched in aconventional manner to form an unetched layer of polysilicon 156covering substantially the second LTO layer 154. A hole 158 may beprovided through the polysilicon layer 156 to the LTO layers 150, 154thereunder by the aforementioned patterning and etching process of FIG.7(f). A post annealing process is performed to render the membranesection of polysilicon 156 in tension. In the present embodiment, thestructure of FIG. 7(f) is put in an oven and heated for approximately 30minutes at approximately 900° C. which changes the crystalline makeup ofthe polysilicon to provide for stress modification thereof.

In the step of FIG. 7(g), the LTO layers 150 and 154 under thepolysilicon layer 156 are removed by a conventional BHF etching processwherein the BHF is allowed to flow through the hole 158 and etch the LTOlayers under the polysilicon layer 156 which are released in solutionthrough the same hole 158. Accordingly, a polysilicon diaphragm 156 intension is produced. Next, the hole 158 in the polysilicon diaphragm issealed by growing a low temperature oxide layer over the hole in aconventional furnace environment.

Next, in the step of FIG. 7(h), a polymer layer 160 which may be aphotosensitive polyimide, a photoresist material, PMMA, or the like, isdeposited over the Si₃N₄ layer 146 of the top surface 142. Patterning ofthe polymer layer depends on the type of polymer used. For example, if apolyimide is used, conventional photolithography may be used to form theannular winding pattern of the inductor coil 124. The patterned portionsof the polyimide are etched conventionally down to the Si₃N₄ layer 146to provide grooves 162 in which to plate the metallic material of theinductor coil 124 within the polyimide layer 160 on the Si₃N₄ layer 146as shown in FIG. 7(i). Preferably, the metallic plating is performedelectroless, but electroplating may also be used without deviating fromthe principles of the present invention. One groove 164 in the polyimidelayer 160 goes down to the annulus of the polysilicon layer 156 so thatwhen plated, the metal end of the inductor coil 124 will make contactwith the polysilicon 156 which is one side of the capacitive element ofthe sensor 10. In addition, a hole may be provided through the Si₃N₄layer 146 at the groove 166 of the other end of the inductor coil 124 toallow the plated metal in the groove 166 to pass through the hole andmake contact with the silicon substrate 140, which is the other side ofthe capacitive element, thus completing the tank or oscillatory circuit.As shown in FIG. 7(j), a thin layer of non-conducting material may begrown over the metallic plated surfaces 172 of a non-compliant region toensure against the sections of coil making contact with each other overthe surface of the polyimide layer 160.

While the present MEMS sensor 51 is described as being fabricated on asilicon substrate, it is understood that other substrates may be usedsuch as a polymeric material, including plastics and polymer films, forexample. Such an alternate MEMS sensor 51 could be fabricated using awell-known micro-replication process such as is illustrated in FIGS.8(a)-8(d), with the simultaneous fabrication of two of the sensors 51being shown side by side. In FIGS. 8(a 1) and 8(a 2), a thin film ofplastic or polymer is mechanically patterned, preferably with dimplesthat would represent wells 54, by a conventional process. The film 52would then be metalized to form a ground electrode 56. A second film 58(FIG. 8(b 1) could be metalized in a pattern to form an inductor 60 andcapacitor (tank circuit). The two films 52 and 58 are then aligned andultrasonically bonded together. Following a final metallization step(FIG. 8(d)) in which a metal is passed through a hole 59 in the secondfilm 58 to form interconnecting conductors 61, the tonometer sensor 51has a structure similar to the structures described herein above for asilicon substrate, but made from a plastic or polymer film instead.

Referring now to FIG. 9, an apparatus 180 that uses the sensor 10 tomeasure IOP is illustrated. The apparatus 180 comprises a contact lens40 having an inner surface 42 contoured to the surface portion 34 of theeye 36. The contact lens 40 may be made of hydrogel or other suitablematerial. The sensor 10 is disposed in the inner surface 42 of thecontact lens 40 so that the contact surface 14 faces the surface portion34 of the eye 36. FIG. 10B illustrates that the sensor 10 is mountedoff-center in the contact lens 40. The weight of the sensor 10 helps tomaintain the contact lens 40 in the orientation shown in FIGS. 9 and10B.

The sensor 10 may be incorporated into the contact lens 40 at the innersurface 42 during the lens fabrication process. For example, if thecontact lens 40 is made using a spin casting process, the lens solutionis injected onto a spinning mold (not shown), with the spin rate andtime being typically computer controlled. The sensor 10 may be placed ina pocket machined into the mold and held in place via vacuum. When themolding is complete, the vacuum is removed from the sensor 10, thecontact lens 40 is removed from the mold and the contact lens with thesensor incorporated therein is handled using conventional procedures.Accordingly, the contact lens 40 including the sensor 10 may be aseparate article of manufacture in accordance with one aspect of thepresent invention

The apparatus 180 further comprises a hand-held eyepiece 182 with arelatively movable applanator 184 for manually applying force againstthe sensor 10 as described further below. The eyepiece 182 includesupper and lower arcuate ridges 184 and 186 for aligning the eyepiece inthe patient's eye socket. The eyepiece 182 further includes an antenna187 (shown schematically in FIG. 10A) for transmitting to and receivingelectrical signals from the tank circuit on the sensor 10.

The applanator 184 resembles a plunger disposed in a cylinder and has adistal end 185. The distal end 185 is movable toward the eye 36 relativeto the eyepiece 182 by pushing manually on a pushbutton mechanism 188.Internally, the motion of the applanator 184 may be opposed or biased bya spring (not shown) and/or a damper (not shown). Further, it iscontemplated that movement of the pushbutton mechanism 188 maypressurize a balloon (not shown) inside the applanator 184 that causesthe distal end 185 of the applanator to move toward the eye 36.Similarly, a bladder (not shown) of silicone gel could be compressedinside the applanator 184 by pressing the pushbutton mechanism 188 tocause the distal end 185 to move toward the eye. It is also contemplatedthat the applanator 184 could include a motorized and/or automatedmechanism that is actuated by pressing the pushbutton mechanism 188 andwhich presses the distal end 185 against the eye 36.

As may be seen in FIG. 9, the applanator 184 projects outward at anangle from the eyepiece 182. The angle at which the applanator 184projects is designed to place the distal end 185 perpendicular to theplane that the sensor 10 lies in when the contact lens 40 is positionedproperly in the eye 36. As is discussed further below, the distal end185 of the applanator 184 is used to press the contact surface 14 of thesensor 10 against the eye to obtain IOP measurements.

When the contact surface 14 of the sensor 10 is pressed against thesurface portion 34 of the eye 36, the response of the sensor 10 overtime is shown in the illustrations of FIGS. 11A1 through 11E2. Each ofthe FIGS. 11A through 11E provides an illustration of the position ofthe sensor 10 in relation to the eye 36 and a corresponding time graphof a pressure representative signal vs. time. The darkened region alongeach time graph is the time interval represented by the respectiveillustration. In FIG. 11A, advancing the sensor 10 toward the cornea 46of the eye 36 causes the sensor to flex. In FIG. 11B, the compliantregion 18 of the sensor 10 initially meets the surface portion 34 of theeye 36. The initial dip in pressure at point 60 from the base linepressure point 62 may be due to surface tension attracting the diaphragm20 of the compliant region 18 just before actual contact with thesurface portion 34 of the eye 36.

Accordingly, as the sensor 10 is pressed further against the surfaceportion 34 and the diaphragm 20 is depressed as shown in FIG. 11C, thepressure representative signal will continue to increase. As theflattening of the surface portion 34 increases, the sensed pressurepeaks, as shown at point 64 in FIG. 11D, starts to decrease as a resultof the bending forces of the cornea 46 being transferred from thecompliant region 18 to across the non-compliant region 16 of the sensor10. Point 64 represents the initial crest of the pressure representativesignal. As the sensor 10 is pressed further against the surface portion34 as shown in FIG. 11E, the pressure reaches a minimum at point 66 andthis minimum represents the IOP of the eye 36. Thereafter, as the sensor10 is moved farther toward and against the surface portion 34, thepressure increases beyond the IOP stage due primarily to an artificialelevation of IOP resulting from additional applanation and other forcesin the eye 36, such as, surface tension from tearing shown at point 68,bending force shown at 70, and tissue tension shown at point 72, forexample. After the IOP has been measured via the sensor 10, the sensoris returned back to its original starting position by the pushbuttonmechanism 188, and the pressure reading is baselined at point 62. Thesensor 10 is then ready to take another IOP measurement.

In order to take the IOP measurements from the sensor 10, a control unit50 (FIGS. 10A and 12) is provided and is operatively coupled, in amanner not shown, to the antenna 187 in the eyepiece 182. The controlunit 50 generates the activation signal for energizing the impedanceelement of the sensor 10 to measure a signal representative of the IOP.This activation signal is preferably an electromagnetic signal thatvaries over a predetermined radio frequency range say from one hundredto two hundred megahertz (100-200 MHz), for example, that energizes thetank circuit of the sensor 10 and causes it to resonate. The controlunit 50 may also include a circuit to detect the resonant frequency ofthe sensor's tank circuit which is proportional to the IOP as shown bythe graph of FIG. 5B, for example. This activation signal may betransmitted from the control unit 50 multiple times over a predeterminedtime interval during which the sensor 10 is in contact with the eye 36.Each electromagnetic activation signal is ramped from a startingfrequency f₁ to an ending frequency f₂ in order for a resonant frequencyto be determined which is representative of a pressure measurementsampling point during the application of the sensor 10 to the eye 36.The collection of this pressure measurement data (or sampling points)provides for a pressure vs. time graph, as exemplified by FIG. 11E, inorder to determine the minimum or actual IOP.

A schematic block diagram of the control unit 50 for use in of thepresent invention is shown in FIG. 12. Referring to FIG. 12, a circuit200 may be triggered by a signal 202 to generate a linear ramping signal204 which ranges from voltages V1 to V2 over a predetermined timeinterval Δt, on the order of 1 millisecond, for example. At the end ofthe time interval Δt, the voltage returns to a predetermined voltagesetting to wait for the next trigger signal over line 202. The linearramping signal 204 governs a voltage controlled oscillator (VCO) circuit206 to generate a sinusoidal signal 208 which overlaps the frequencyrange of the sensor 10 as the signal 204 ramps from V1 to V2. The signal208 may be amplified by a radio frequency (RF) amplifier circuit 210which drives a resistor/inductor series combination, R1 and L1,respectively. The output of the RF amplifier 210 may be provided to apulse shaper circuit 212 over signal line 214 which in turn is coupledto a cascaded pair of digital counters 216 and 218. The digital outputof counter 218 is captured in an output buffer 220.

The voltage across the inductor L1 is input to another RF amplifier 222via signal line 224. The output 226 of the RF amplifier 222 is providedto a root-mean-square (RMS) detector 228, the output 230 of which beingcoupled to a comparator circuit 232. In the present embodiment, thecomparator circuit 232 functions as a signal peak or valley detector andgenerates a signal over line 234 when the signal peak or valley isdetected. The signal line 234 is coupled to the counter 218 and outputbuffer 220 for operation thereof. The circuits of the control unit 50may be centrally controlled in operation by a digital controller 240,which may be a programmed microprocessor, digital signal processor or acombination of hardwired digital logic circuits. A memory unit 242 iscoupled to the digital controller 240 and may be comprised of acombination of static, dynamic and read-only memory units, for example,for the storage of data and program information. A switch 244 is coupledto the digital controller 240 through conventional input-outputcircuitry (not shown). The digital controller 240 may also be coupled toa conventional display unit 246 for displaying IOP readings. The controlunit 50 may also include an upload/download circuit 250 for transmittingdata between the digital controller 240 and an external computer, like aPC, for example, over a hardwired connection.

Taking an IOP reading using the sensor 10, including the apparatus 180and the control unit 50, will now be described in connection with FIGS.9, 10A, 10B, 11E, and 12. With the contact lens 40 positioned in the eye36 as shown in FIG. 9, the eyepiece 182 is brought into engagement withthe patient's eye socket. This provides a rough alignment of the distalend 185 of the applanator 184 with the sensor 10 in the contact lens 40.This alignment is important because only localized pressure on thecontact lens 10 is desired, as pressure applied to the entire cornea 46may result in artificially high IOP measurements.

With the patient's eyelids 190 closed, as may be seen in FIG. 9, thepushbutton mechanism 188 is manually pressed until the distal end 185 ofthe applanator 184 presses firmly against the eyelid which, in turn,causes the contact surface 14 of the sensor 10 to firmly engage thesurface portion 34 of the eye 36.

As the applanator 184 is being moved toward the eye 36 as shown in FIG.11A1, the switch 244 may be depressed for taking an IOP reading. Inresponse to the depression of the switch 244, the digital controller 240commences with a sequence of control operations to perform the IOPreading. Trigger signals are generated at predetermined times oversignal line 202 to cause the linear ramp circuit 200 to generate theramping signals which controls the VCO circuit 206 to drive the inductorL1 via RF amplifier circuit 210 and resistor R1. In turn, the inductorL1 is coupled magnetically to the inductor of the sensor 10 andelectromagnetically activates and drives the tank circuit of the sensor.As has been described herein above, the capacitive element (compliantregion 18) of the sensor 10 will change in impedance as it is forcedagainst the surface portion 34 of the eye 36. This change in impedancewill cause a change in circuit resonance. Sensor readings are thus takenat the points of resonance of the magnetically coupled circuits. Morespecifically, during the time interval of each frequency ramp, the RMSvoltage across the inductor L1 is monitored by the circuits 222, 228,and 232 to establish the point in time of resonance. At resonance, asignal is generated by the comparator circuit 232 to the digitalcontroller 240, the counter 218, and the output buffer 220. In responseto this signal, the digital count of the counter 218 which isrepresentative of the resonance frequency is captured in the outputbuffer 220 and subsequently, read by the controller 240 and stored inthe memory 242. When the digital count has been read and stored, thecontrol unit 50 may generate an audible signal indicating that ameasurement has been taken, and the process may then be repeated. Thestored digital counts of each of the frequency sweep time intervalsrepresent sampled data points which together form the pressure profileof FIG. 11E. The digital controller 240 then processes these sampleddata points to determine the current IOP reading, which may be day andtime stamped and stored in the memory 242 and displayed in the digitaldisplay 246.

FIGS. 13 and 14 illustrate an alternate embodiment of the presentinvention in which the patient's eyelids 110 are open and the distal end185 of the applanator 184 directly engages the contact lens 40 to applypressure. In this embodiment, an aperture 192 is formed in the eyepiece182 for the patient to look through.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. For example, itis contemplated that the applanator 184 could be disposed on the end ofan instrument in a doctor's office, rather than a hand-held unit. It isfurther contemplated that other physical configurations of theapplanator 184 could be used, such as a finger-mounted device whichwould, of course, include the antenna 190. Finally, it is conceivablethat closed eyelids 190 may be able to supply sufficient pressure ontheir own to press the sensor 10 against the eye 36, in which case theeyepiece 182 would carry only the antenna 190 and not the applanator184. Such improvements, changes and modifications within the skill ofthe art are intended to be covered by the appended claims.

Having described the invention, we claim:
 1. An apparatus for measuring intraocular pressure of an eye, said apparatus comprising: a contact lens including an inner surface contoured to a surface portion of the eye for engaging the surface portion; and a sensor disposed in said inner surface of said contact lens, said sensor comprising: a contact surface for making contact with the surface portion of the eye, said contact surface including an outer non-compliant region and an inner compliant region fabricated as an impedance element that varies in impedance as said inner compliant region changes shape; and a region of conductive material electrically coupled to said impedance element of said compliant region and responsive to an external signal for energizing said impedance element so that the intraocular pressure may be determined.
 2. The apparatus of claim 1 wherein said sensor is comprised of silicon material.
 3. The apparatus of claim 1 wherein said sensor is comprised of a polymeric material.
 4. The apparatus of claim 3 wherein said sensor comprises at least two layers of polymeric film bonded together.
 5. The apparatus of claim 1 wherein said compliant region comprises a diaphragm that functions as one plate of a capacitive element, said diaphragm being separated by a dielectric region from another plate of said capacitive element, said diaphragm flexing closer to said other plate as said contact surface is pressed against the surface portion of the eye to change the capacitance of said capacitive element in proportion to the intraocular pressure of the eye.
 6. The apparatus of claim 5 wherein said dielectric region comprises air.
 7. The apparatus of claim 5 wherein said dielectric region comprises hydrogel.
 8. The apparatus of claim 5 wherein said dielectric region comprises silicone.
 9. The apparatus of claim 5 wherein said region of conductive material comprises an inductor coil that is electrically coupled to said capacitive element to form a resonant circuit, the external signal comprising an electromagnetic signal that varies in frequency to cause said resonant circuit to be energized and resonant at a frequency in proportion to the capacitance of said capacitive element so that the intraocular pressure may be determined.
 10. The apparatus of claim 9 wherein said inductor coil is fabricated in said non-compliant region.
 11. The apparatus of claim 9 wherein said inductor coil is fabricated on said inner compliant region.
 12. The apparatus of claim 11 further comprising a second inductor coil formed underneath said diaphragm.
 13. The apparatus of claim 9 wherein said inductor coil is formed by disposing conductive material in a predetermined pattern in a surface of said non-compliant region about said compliant region of said contact surface.
 14. The apparatus of claim 9 further comprising an applanator for applying pressure against said contact lens to cause said contact surface of said sensor to firmly engage the surface portion of the eye.
 15. The apparatus of claim 14 further comprising an eyepiece for covering over the eye, said applanator being mounted in and movable relative to said eyepiece.
 16. The apparatus of claim 15 further comprising an antenna disposed on said eyepiece, said antenna for transmitting the external signal for energizing said impedance element.
 17. The apparatus of claim 16 further comprising a control unit for generating the external signal to measure a signal representative of intraocular pressure, said control unit being operatively coupled with said antenna.
 18. The apparatus of claim 17 wherein said compliant region comprises a capacitive element that changes capacitance in proportion to a change in shape, said region of conductive material comprising an inductive coil electrically coupled to said capacitive element to form a resonant circuit.
 19. The apparatus of claim 18 wherein said control unit generates an electromagnetic signal that varies over a predetermined frequency range to cause said resonant circuit to resonate, said control unit including means for measuring the resonant frequency of said resonant circuit which is representative of the intraocular pressure of the eye.
 20. The apparatus of claim 17 wherein said control unit includes processing means for measuring signals representative of intraocular pressure at different times during a predetermined time interval, and a memory for storing the measured signals representative of the intraocular pressure measured at said different times.
 21. The apparatus of claim 20 wherein said control unit includes means for processing the stored measured signals representative of intraocular pressure to determine a resultant intraocular pressure (IOP) measurement.
 22. The apparatus of claim 21 wherein said control unit includes means for time marking each resultant IOP measurement with a measurement time and for storing said resultant IOP measurements with their corresponding measurement times in the memory.
 23. The apparatus of claim 22 wherein said control unit includes means for transferring the stored resultant IOP measurements and their corresponding measurement times to another system.
 24. The apparatus of claim 17 wherein said control unit includes a display for displaying the intraocular pressure measurements.
 25. A method for measuring intraocular pressure (IOP) of an eye, said method comprising the steps of: providing a contact lens having an inner surface contoured to a surface portion of the eye, the contact lens including a sensor disposed in the inner surface of the contact lens, the sensor having a compliant region that functions as an impedance element; positioning the contact lens on the surface portion of the eye; providing an applanator for applying pressure against the contact lens; moving the applanator toward the eye until the sensor forcefully engages the surface portion of the eye which causes the compliant region to change shape and vary in impedance; energizing the impedance element; determining a representative pressure measurement each time the impedance element is energized; and processing the representative pressure measurements to render a resultant IOP measurement.
 26. The method of claim 25 wherein said step of energizing the impedance element includes the step of: energizing an inductive region of the sensor that is connected to the impedance element which is a capacitive region to cause a circuit formed by the regions to resonate.
 27. The method of claim 26 wherein said step of energizing an inductive region of the sensor includes transmitting an activation signal over an antenna.
 28. The method of claim 26 wherein said step of energizing the impedance element includes generating an electromagnetic signal with a frequency that is swept through a frequency range over a predetermined time interval, the resonant frequency of the circuit falling within said frequency range.
 29. The method of claim 28 wherein said step of determining includes the steps of determining the resonant frequency of the circuit each time the circuit is energized, the resonant frequencies sampled being representative of the IOP of the eye at different times.
 30. The method of claim 29 wherein said step of processing includes processing the sampled resonant frequencies to render a resultant IOP measurement.
 31. The method of claim 30 further comprising the steps of: time marking each resultant IOP measurement; and storing each IOP measurement along with its corresponding measurement time.
 32. The method of claim 31 further comprising the step of transmitting the stored IOP measurements and their corresponding measurement times to an external site.
 33. The method of claim 32 wherein said steps of energizing, determining and processing are performed autonomously by a control unit.
 34. The method of claim 33 further comprising the step of displaying the resultant IOP measurement on the control unit. 